It was an American, in 1919. At _________ time, the bags were mainly used by tea companies to allow customers to try the tea before buying it.

• A. That
• B. Which
• C. When
• D. Where

Answer: (A)

Refer back to a specific moment using a demonstrative before the noun. “At that time” points to the moment already mentioned (in 1919) and is a natural, common way to connect the past reference to what followed. The other options don’t fit as smoothly: “at which time” is grammatically possible but sounds formal and less natural here, while “at when time” and “at where time” aren’t correct English. So the natural choice is “that,” giving the phrase “At that time.”